The present invention relates to an off-line switched mode control system with frequency jittering.
Many products rely on advanced electronic components to cost-effectively provide the product with the desired functionality. These electronic components require power regulation circuitry to supply them with a clean and steady source of power. The development of switched mode power supply technology has led to power supplies operating at high frequency to achieve small size and high efficiency. Each switched mode power supply typically relies on an oscillator switching at a fixed switching frequency or alternatively a variable frequency (such as in a ringing choke power supply).
Due to the high frequency operation relative to the frequency of an alternating current (AC) power line, switched mode power supplies can exacerbate problems associated with electromagnetic interference (EMI). EMI noise is generated when voltage and current are modulated by the switching power supply. This electrical noise can be transferred to the AC power line.
In addition to affecting the operation of other electronics within the vicinity of the power supply by conduction, EMI induced noise on a power line may radiate or leak from the power line and affect equipment which is not even connected to the power line. Both conducted and radiated electrical noise may adversely affect or interfere with the operation of the electronic equipment. For example, EMI noise generated by the switching power supply can cause problems for communication devices in the vicinity of the power supply. Radiated high frequency noise components may become a part of the AC mains signal and may be provided to other devices in the power grid. Further, power supply radiated EMI can interfere with radio and television transmissions.
To address EMI related interference, several specifications have been developed by government agencies in the United States and in the European Community. These agencies have established specifications that define the maximum amount of EMI that can be produced by various classes of electronic devices. Since power supplies generate a major component of the EMI for electronic devices, an important step in designing such supplies that conform to the specifications is to minimize EMI emission to the acceptable limits of the various specifications.
EMI may be reduced in a power supply by adding snubbers and input filters. These components reduce the noise transferred to the power line and by so doing, also reduce the electric and magnetic fields of noise generated by the power line. While these methods can reduce EMI, they usually complicate the design process as well as increase the production cost. In practice, noise filtering components are added in an ad hoc manner and on a trial-and-error basis during the final design process when EMI is found to exceed the compliance limits specified by the regulatory agencies. This inevitably adds unexpected costs to the products.
Further, extra components can undesirably increase the size and weight of the power supply and thus the resulting product.